The present invention relates to a process for the surface treatment of individual particles of a pulverulent material to improve the surface properties thereof. The invention also covers a plasma generating apparatus for use in such a process.
An important aspect in modern technology is constituted by the modification of the surface properties of different materials. Such modification can be obtained by for example treatment by the application of active species to the surface or by applying a coating to the surface. In recent years much attention has been directed to developing technologies enabling surface treatment of powders of different kinds. Particularly, the treatment of fine-sized pulverulent materials is subject to severe problems when trying to obtain uniform treatment of the surface of each individual particle to improve or modify the properties thereof.
One severe problem arising in connection with the surface treatment of pulverulent materials resides in the fact that the fine particles agglomerate to form clusters or aggregates, whereby the treatment will be inefficient and not reaching all exterior surfaces of the particles to be treated. Another problem resides in the fact that powders are difficult to handle in transportation due to dusting, irregularities in flow, fluidization, etc.
Different techniques have been developed for the purpose of improving the properties of powders of different kinds, and among such technologies there may be mentioned improvement in flow in iron or steel powder technology by admixing organic lubricants, and the treatment of organic pigments for the purpose of imparting a polarity to the surfaces of the particles thereof. Thus, U.S. Pat. No. 4,478,643 describes techniques for low temperature plasma treatment of organic pigments to obtain such polarity.
However, all technologies hitherto developed are subject to drawbacks, mainly due to the fact that they do not permit uniform modification of the surface properties of the particles treated mainly due to agglomeration phenomena. Moreover, the technologies devised up to now do not permit treatment of different types of powders and known techniques are specifically directed to the treatment of specific materials.
The present invention has for its main object to provide new techniques for the surface treatment of pulverulent materials.
Another object of the invention is to provide a process for the surface treatment of the individual particles of a pulverulent material to improve the surface properties thereof.
Another object of the invention is to provide techniques using a low temperature plasma flame in the treatment of the individual particles of a pulverulent material.
Yet another object of the invention is to provide techniques enabling efficient surface treatment of powder particles irrespective of the material on which said powder is based.
The present invention is based on the surprising discovery that if the pulverulent material to be treated is injected into an inert gas used for generating the low temperature plasma flame efficient disintegration of any agglomerated particles present in the material will be obtained if the inert gas is travelling at a high velocity when contacted by the pulverulent material. High disintegrating efficiency will be obtained if the inert gas travels at a high drift velocity and uniform surface treatment will be obtained by using such techniques.
Accordingly, the present invention provides a process for the surface treatment of individual particles of a pulverulent material to improve the surface properties thereof. The process is comprised by the following steps:
a) generating a plasma in a plasmagenerator while supplying an inert gas at a high velocity into said generator to form a low temperature plasma flame which is directed into a low pressure zone;
b) injecting the pulverulent material into said high velocity inert gas or into the base of said plasma flame to disintegrate any particle aggregate present in said material;
c) allowing the low temperature plasma to act on the surface of each individual particle to improve the properties thereof; and
d) recovering the treated particles from said zone.
According to one aspect of the invention, powders can be treated for modifying the surface properties thereof by treatment with non-polymer forming gases, such as ammonia, oxygen, water vapor, etc.
According to a particularly preferred embodiment of the process of the present invention the surface treatment involves the formation of a uniform surface coating on each individual particle using plasma polymerization. In obtaining such coating step b) above is accompanied by the step of injecting a monomeric gas into the inert gas at a first location upstream of the plasma flame. Such monomeric gas may be constituted by one or more monomers.
As indicated above, efficient and uniform surface treatment of the individual particles of the pulverulent material will be obtained if the drift velocity of the inert gas used to form a low temperature plasma flame is high and exceeds about 100 m/sec. Thus, it is preferred that the inert gas drift velocity is higher than about 100 m/sec., such as higher than about 200 m/sec. and especially higher than about 300 m/sec. Drift velocity is calculated according to the formula: ##EQU1## where F is the flow rate of the gas, A is the cross-section of the flow passage and P is the pressure. It can be seen from said formula that the lower the pressure, the higher the velocity.
Although the process can be performed using any type of low temperature plasma generator a preferred type of generator is a so called cascade arc plasma torch generator. For details concerning such apparatus reference is made to co-pending U.S. patent application Ser. No. 274,775, the disclosure of which is incorporated herein by reference.
When using plasma polymerization to form a uniform surface coating on each individual particle it is preferred to supply the pulverulent material at a location which is downstream of the location at which the monomeric gas is supplied. According to another aspect of the invention the pulverulent material may be entrained in the monomeric gas supplied or in a part thereof.
As indicated earlier in this disclosure the process of the present invention is unique in that it allows efficient surface treatment of the individual particles of a pulverulent material irrespective of the material subject to treatment. Thus, the pulverulent material may be constituted by metal or metal alloy powders, metal compound powders, organic powders or ceramic powders. Particularly interesting materials are iron or steel powders used in metal powder technology, and metal oxides. Another interesting group of materials are organic or inorganic pigments. A third group of materials are metal oxides, such as titanium oxide or zirconium oxide, where titanium oxide is of interest when used dispersed in polymeric materials to allow for determination of location using X-ray techniques. A further group is constituted by hard metals, such as tungsten and titanium carbides. Further materials are aluminum powders, aluminum hydrate powders used for flame retardation for dispersion in polymers, SiO.sub.2 for example used as a filler, etc.
The monomeric gas used if plasma polymerization is applied may be constituted by hydrocarbons and halogenated hydrocarbons, silanes, organosilanes and organometallic compounds, optionally together with hydrogen, H.sub.2 O or chemically reactive gases. Examples of organometallic compounds are tetramethyl tin and di-ethyl zinc. For details concerning such monomeric gas reference is made to the above-mentioned co-pending U.S. patent application.
The invention also covers a plasma-generating apparatus for use in the low temperature plasma treatment of the surface of the individual particles of pulverulent materials. Such apparatus comprises means for generating a vacuum zone and, in association therewith, a plasma generator having a central passage therethrough opening into said vacuum zone. The apparatus includes means for introducing an inert gas into said passage and is characterized by powder inlet means for introducing a pulverulent material to be treated into said passage and by discharge means for removing treated material from the vacuum zone. To maintain an efficient vacuum in the vacuum zone it is preferred that the discharge means are designed as lock means for intermittent discharge of treated pulverulent material.
In accordance with a preferred aspect of the invention the plasma-generating apparatus further comprises inlet means located upstream of the powder inlet means for introducing monomeric gas enabling plasma polymerization for the purpose of forming a uniform surface coating on each individual particle in the vacuum zone.